It then informed me that I had 1120 updates !
I'm not very knowledgable on the software update process but obviously this didn't look right.
I had 2 Mint Updates last week - I think one was the 'mint-debian mirrors' TomRoche referred to
(Is there a way of checking what was updated ?)

Panic Over!
Looks like I hit problems the start of Update 5, which I didn't know was coming.
It's now showing 1409 updates which I'll load at a quiet time (when USA and Europe are asleep ?)
now that I've backed up with Clonezilla.
Sorry but it looked nasty at the time
Iain